The present invention relates to a system for precisely determining the location of a cursor on a work surface with respect to a grid of coordinate conductors.
Systems for recording points and curves on a work sheet by monitoring the position of a cursor or similar movable device on a work surface are well known in the art. Such systems are known by various names, such as "coordinate digitizer" or "digital transducer". In general, these systems comprise a work table, known as a tablet, having a conductor grid lying under the surface thereof. From signals coupled between the conductor grid and the cursor, data representing the Cartesian coordinates of the cursor on the work surface are generated. Thus an operator may place a drawing or the like on the work surface an generate and store data representing points or lines on the drawing simply by tracing out the points or lines with the cursor.
In one particular type of system, the tablet has for each axis a plurality of spaced, parallel conductors essentially co-extensive with the work surface. Circuitry produces successive sequential pulse excitation of the conductors, resulting in an effectively moving pulse wave. A cursor, stylus, or other sensor responds to the passage of the pulse wave through the position of the cursor. For example, the cursor may carry an inductive pickup coil, and a voltage is induced therein as the pulse wave passes. The position of the cursor along the coordinate axis may be determined from the precise time of passage of the pulse wave through the cursor position. In a typical arrangement, a high resolution timer comprising a digital counter determines the time between the beginning of the conductor excitation sequence and the response by the cursor. This time then represents the cursor distance along the coordinate axis.
It will be appreciated that, insofar as the general concepts of this type of system and of the present invention are concerned, the precise nature of the mechanical arrangement which carries the pickup coil is of little significance, and the term "cursor" as employed herein is intended to include all similar devices which serve to indicate to the operator the particular point on the work surface which is being recorded. This is not intended to imply that there are not differences in the various possible cursor-like mechanical arrangements; only that these differences are not significant with respect to the scope of the present invention.
Examples of this general type of system wherein tablet conductors are sequentially energized are disclosed in the following U.S. Pat. No. 3,342,935--Leifer et al; No. 3,904,822--Kamm et al; No. 3,975,592--Carvey; No. 4,054,746--Kamm; and No. 4,080,515--Anderson.
In another general type of system, a plurality of tablet conductors are energized at the same time with signals which are out of phase. By responding to phase information, and possibly frequency information, the movable cursor sensor and associated circuitry can provide data indicating cursor position. Examples of this general type of system are disclosed in the following U.S. Pat. No. 3,342,935--Leifer et al (also cited above); No. 3,647,963--Bailey; No. 3,983,322--Rodgers; and No. 4,022,971--Rodgers.
While these various prior art devices generally serve their desired function, and at least one has been commercially produced, they do as a class have a number of disadvantages such as undue complexity, being difficult and expensive to manufacture, being difficult or tedious to calibrate, having limited accuracy attainable as a practical matter, and being difficult to repair. Prior to the present invention there therefore remained a need for an improved digital transducer, which need is met by the present invention.